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Officials are only challenging the age of child migrants if they look like they are over 25, The Daily Telegraph has learned as it emerged some of those claiming to be under 18 have already been proven to be adults.

Home Office guidance states that asylum seekers should only be challenged about their age if they are found to be "significantly over 18 years of age".

Staff have been instructed that the policy should only be used with "extreme caution" and that they should only verify their age if their appearance suggests they are "at least 25 years old".

It came as a minister revealed that one in 10 of those are claiming to be children in the Calais camp have been found to be adults.

It also emerged that one of the first child refugees to arrive in Britain has been proven to be an adult. According to reports the man's fingerprints were already on the UK's biometric database, proving that he was older.

The fact he was on the database could mean that he had previously attempted to enter the UK or had a criminal record. The man is not thought to be "significantly" older than 18 and may still be entitled to stay in the UK.

Philip Davies, a Conservative MP, yesterday said in the Commons that ministers risk doing "irreparable damage to public confidence in the asylum system" unless they strengthen age checks. He said that his constituents feel like "we are being taken for fools" and that "our generosity is being abused".

He said: "We agreed to take in child refugees and surely it's not too much to ask of the Government to ensure that they are children and clearly this is not the case.

"People only have to see the pictures of the so-called child refugees to see that many of them are not children."

Robert Goodwill, the migration minister, said that dental checks to verify people's age are both inaccurate and unethical as he ruled out using them.

He said that "teenager's appearance varies widely" and said that the fact his own wisdom teeth "didn't come down until quite late in life" was evidence that dental checks would not work.

He said that 10 per cent of asylum seekers claiming that they are children in Calais have been rejected on the grounds that they are too old.

Mr Goodwill said the voices of a critical "small minority", whether in the media or elsewhere, "should not be listened to" on the issue.

It came as the latest group of refugee children to arrive from the Calais Jungle were kept hidden behind 15 foot screen as they arrived in Croydon, South London.

Earlier this week Conservative MPs accused the Home Office of a "cover up" after refugees hid their faces under blankets as they arrived. It comes after suggestions that some of the first "children" to arrive looked significantly older.

Citizens UK, who organised a "Refugees Welcome" event for their arrival, said the screens were put up at the request of the Home Office to protect the youngsters - rather than to hide their age.

A report by the Chief Inspector of Borders in 2013 disclosed that staff only challenge child migrants about their age if they look like they are aged over 25.

The report states: "We found from staff interviews and our observation of a trainer, that staff were encouraged to use this option in the guidance when perceiving an applicant to be at least 25 years old. "One stakeholder report advocated using this policy ‘with extreme caution’, and we therefore found the Home Office’s use of an eight-year age gap to be reassuring as an example of safeguarding."

David Simmonds, chairman of the Local Government Association's asylum, refugee and migration task group, said the age guidance had been in use for some time.

He said: "In a supermarket, if a person doesn't look over the age of 25, the shop worker will ask for ID before selling them alcohol. But in the case of border control, it works the other way around. They are told to err on the side of trusting that the person is telling the truth unless they look over 25."

The head of the French charity working with the Home Office in Calais yesterday disclosed that 95 per cent of the 1,290 unaccompanied children in the camp want to come to the UK.

Pierre Henry said: “It’s a difficult job but it has not been at all chaotic at our end. However, it is up to the Home Office to decide which children go to Britain,” Mr Henry told the Daily Telegraph.

He said that 95 per cent of the 1,290 unaccompanied minors in Calais wanted to go to Britain. “Forty per cent of them, which means about 500, say they have relatives in the UK.”

The child refugees are coming to Britain because French authorities are demolishing the "jungle" camp in Calais imminently.